Metamask permits

[kent-3]

I've found an existing crate to transform the compact form of JSON string from serde-json-wasm into a "pretty" version, with an indent of 4 spaces. It's using String manipulations, so I don't think there are any floating point issues with this approach, but have not tested uploading this as a contract.

[kent-3]

I decided it would be better to create a custom serde Serializer for this instead. I can either include the 'pretty' module directly in this repo, or add it to a fork of serde-json-wasm and use that as a dependency (like I've done here: https://github.com/kent-3/serde-json-wasm).

[DrPresident]

Looks good from what I can tell, not super familiar with how this encryption works and I didn't scrutinize the pretty-printing in depth. Tests look good, would be nice to add one that actually verifies an ethereum permit.